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# C2 Maths Question Cosine Rule watch

1. Here is the question.

From a point A a boat sails due north for 7km to B. The boat leaves B and moves on a bearing of 100 degree for 10km until it reaches C. Calculate the distance of C from A.

I have been trying this question for the past 30 min and my eeverytime I get 5.3 and the answer is qrong, so i was wondering if someone could help me with this question.

Thanks
2. Draw a diagram first if you haven't already. You have a triangle with two sides 7 and 10 with the enclosed angle of 80 degrees. Line b is what you are trying to find (opposite angle B).

b^2 = a^2 + c^2 - 2ac cos B

b^2 = 100 + 49 - 140 cos 80

I get 11.16643431....
3. (Original post by Shawshank)
Draw a diagram first if you haven't already. You have a triangle with two sides 7 and 10 with the enclosed angle of 80 degrees. Line b is what you are trying to find (opposite angle B).

b^2 = a^2 + c^2 - 2ac cos B

b^2 = 100 + 49 - 140 cos 80

I get 11.16643431....
Why 80 degree why not 100 degree. Could explain please and you answer is right btw
4. You travel on a bearing of 100 degrees. The bearing is measured from the north. 180 degrees in a straight line, 180 - 100 = 80.

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